Submarine optical cables are used to provide high bandwidth data connections over long distances. It is often necessary for a submarine optical cable to provide electrical power, for instance to power repeaters along the cable that maintain the optical signals carried by the cable. Electrical power feed equipment (PFE) may be used to provide electrical power to the optical cable. The electrical power may be provided as direct current, since this is a more efficient method of power transmission over long distances.
The maximum potential of an electrical conductor of a subsea cable is typically limited by the breakdown voltage of the insulation between the conductor and the adjacent seawater (e.g. 12 kV or 15 kV). The power handling capacity of such a cable is therefore maximized if a positive voltage is applied at one end of the cable and a negative voltage is applied at the other end.
Shunt faults may occur, which result in a conducting path between the electrical conductor of the cable and the surrounding seawater. Such a shunt fault may be mitigated by varying the electrical potential at either end of the cable to reduce the voltage in the region of the fault, so that the voltage difference between the cable and the surrounding seawater in the region of the fault is minimal. This requirement to vary the voltage at either end of the cable is in conflict with enabling maximum power handling. Maximum power handling requires double end feeding: that a positive voltage (preferably near the breakdown voltage) is provided at one end, and a negative voltage (preferably near the breakdown voltage) is provided at the other end. Accommodating shunt faults requires sufficient voltage headroom at either end of the cable to enable the voltage in the region of the fault to be minimized.
These conflicting requirements make it difficult to engineer a solution that is reliable and which maximizes power handling (which in turn enables increased system data capacity).